PROJECT SUMMARY The opioid epidemic in the US is associated with a dramatic increase in problematic opioid use and overdoses. Chronic pain affects over 100 million Americans and opioids are frequently prescribed to aid patients in managing pain. Approximately 25% of patients with chronic pain misuse prescription opioids and approximately 10% have opioid use disorder (OUD). Clinical practice guidelines stress the importance of non-opioid medications or non-pharmacological interventions for chronic pain and recent population-based studies suggest that patients use cannabis for pain relief. There is considerable interest in determining whether cannabis and cannabinoid products are viable alternatives to opioids for pain management. However, the potential opioid-sparing effect of cannabis is based primarily on studies of correlational findings that are limited by retrospective recall bias. Researchers have noted the need for intensive longitudinal studies on near real-time co-use of these substances and pain symptoms in the natural environment. A novel method to evaluate opioid and cannabis use patterns among patients with chronic pain is Ecological Momentary Assessment (EMA). EMA is ideally suited to collect fine- grained data on substance use and pain, with low recall bias and high ecological or real-world validity. The overarching goal of this R21 exploratory research grant application is to conduct a smartphone-based real-time observational study using EMA to investigate the relationship between opioid use, cannabis use, and chronic pain. We have assembled a strong team of investigators with critical expertise in EMA, opioid and cannabis research, and chronic pain. Our preliminary studies have shown that we can conduct EMA studies focusing on substance use and co-use with different populations. Moreover, we have demonstrated that many cannabis users report chronic pain as the most important reason for their cannabis use. We have also demonstrated that there is substantial day-to-day variability in pain and opioid use among patients with chronic pain and may be influenced by factors other than their prescriptive plan. The specific aims of this project are to: 1) Develop an EMA data collection protocol and investigate participant adherence among chronic pain patients who are using opioids and cannabis. 2) Explore the temporal interplay of cannabis and opioid use on a day-by-day level and associations between use of these substances and pain ratings. To attain these aims, we will conduct a study consisting of 2 phases. Phase 1 will conduct cognitive interviews to refine EMA measures and pilot a 14-day EMA data collection with N=20 participants, including follow-up interviews. EMA questions will be refined and clarified based on participant feedback. Phase 2 will consist of a 30-day EMA data collection with N=100 chronic pain participants.